Such a hammer which is known from EP 1 223 010 A1 may be used to conduct demolition works wherein a tool bit formed as a chisel is usually driven into the material of the work piece. In addition, it can be conceived that the hammer is configured as a hammer drill having a tool holder which is also rotationally driven. In this case a drill bit may be used as a tool bit rather than a chisel.
In such a hammer the ram is usually driven in such a way that in addition a piston is guided within the cylinder wherein an air cushion is provided between the piston and the ram. The piston is coupled with a crank drive so that the rotational movement of the motor shaft is converted into a reciprocating movement of the piston. This movement in turn is transferred to the ram via the air cushion, the ram hitting either directly the tool bit supported by the tool holder or a beat piece arranged between the ram and the tool bit wherein in both cases the momentum of the ram is transferred to the tool bit.
The magnitude of the momentum of the ram depends on the acceleration which has been received by the ram due to the movement of the piston. The velocity of the piston in the direction of the longitudinal axis of the cylinder depends in turn on the rotational speed of the motor shaft of the motor. As a consequence, the power of impact the ram applies to the tool bit, depends on the rotational speed of the motor and is relatively low if the motor runs at low speed. Further, in case of such a hammer changing the rotational speed of the motor is the only way for changing the power of impact of the ram. However, it may be desirable to provide an increased power of impact even at low rotational speed, if the hammer is used in the drill mode, i.e. the cylinder with the tool holder thereon is rotatingly driven.
Therefore, it is the object of the present invention to provide for a motor driven hammer which allows for a change of the power of impact which is transferred from the ram to the tool bit without the need for changing the rotational speed of the motor.